Utility location

Public utility systems are often run underground—some by the very nature of their function, others for convenience or aesthetics. Before digging, local governments often require that the underground systems' locations be denoted and approved, if it is to be in the public right-of-way.

Because of the many different types of materials that go into manufacturing each of the different types of underground lines, different detection and location methods must be used. The two general methods are called passive locating and active locating.[2]

Passive locating uses signals that occur naturally on some utility lines. This method often fails to locate 60% or so of utility lines, and misidentifying utilities is an associated risk. Active locating, on the other hand, is more accurate. It makes use of signals that the locators generate through a transmitter. Then, with the help of a receiver, locators can trace as well as identify what type of utility line lies underground.[2]

The active method is the most suitable for locating metal pipes and cables. This process often involves using a piece of electromagnetic equipment, consisting of a transmitter and a receiver. For other types of pipe, such as plastic or concrete, other types of radiolocation or modern ground-penetrating radar must be used. Location by these technical means is necessary because maps often lack the pinpoint precision needed to ensure proper clearance. In older cities, it is especially a problem since maps may be very inaccurate, or may be missing entirely.

A few utilities are permanently marked with short posts or bollards, mainly for lines carrying petroleum products. This may be done because of venting requirements, and also serves to indicate the location of underground facilities that are especially hazardous if disturbed.

Locating underground utilities within the vicinity of an area is also a must. This vicinity, also called the tolerance zone,[3] usually includes between 18 to 24 inches from the ends of a utility line. The specific distance varies from state to state, so it is a must for locating professionals to confirm the specifics with the State that covers their work area.[3]

"Call before you dig", "Know What's Below! Dig With Confidence!", "Dial before you dig", "Digger's Hotline", "One-call", "Miss Utility", "Dig Safe", "Click before you dig", or Underground Service Alert are services that allow construction workers or homeowners to contact utility companies, who will then denote where underground utilities are located via color-coded markings typically up to and not past the user's service connection or meter. Lines on the other side of the meter or service connection are considered to be part of the customer's property and will typically not be located by these services.

Failure to call such a number ahead of time may result in a fine or even a criminal charge against a person or company, particularly if such negligence causes a major utility outage or serious accident, or an evacuation due to a gas leak. Hitting a water main may also trigger local flooding or require a boil-water advisory.

The following are the partial list of countries with one-call for utility location:

• Australia: The national "dial before you dig" number is 1100.[4]
• Canada: In Canada, there is no unified number for the country. However, it has the "Click Before You Dig" website that provides access to the hotline for each of the provinces.[5]
• United States: As required by law and assigned by the US Federal Communications Commission (FCC), the 8-1-1 telephone number is used for this purpose across the United States.[6]
• United Kingdom: In Scotland there is an online web based service called Vault[7] (short for Scottish Community Apparatus Data Vault), which allows registered users to specify a polygon on a map and immediately obtain an interactive map of all major undertakers owning underground assets, except the principal telecoms operators). In the rest of the UK, there is no dial-before-you-dig service. Instead the onus is on excavators to ensure that they do not damage buried services.[8] This has resulted in a class of locating tools which are simpler to use, and are called Cable Avoidance Tools (CATs), and the associated signal detectors are called Gennys.

Yellow spray paint indicating below-street gas utility line (New York City, 2007)

Colored flags for utility locating

Utility color codes are used to identify existing underground utilities in construction areas, to protect them from damage during excavation. Colored lines, flags, or both are used to mark the location and denote the type of underground utility. A special type of spray paint, which works when the can is upside-down, is used to mark lines, often in a fluorescent color. On flags, a logo often identifies the company or municipal utility which the lines belong to.

Flags may also be an advertisement for a company which has installed an irrigation system for lawns or gardens. In this case, each sprinkler head is usually marked, so that landscaping crews will not cover or bury them with soil or sod, or damage them with tractors or other construction equipment while digging holes for trees, shrubs, or other large plants or fenceposts. This is also important because a vehicle (tractor, truck, or otherwise) can break a sprinkler or the hard-PVC pipe or joint it is mounted on, simply by driving over it, particularly on newly moved soil which is uncompacted and therefore unsupportive of such weight.

Canada & United States

Utility markings

Canada and the United States use the American Public Works Association (APWA) Uniform Color Codes for temporary marking of underground utilities:[11][12][13][14][15]

Red	electric power lines, cables, conduit, and lighting cables
Orange	telecommunication, alarm or signal lines, cables, or conduit
Yellow	natural gas, oil, steam, petroleum, or other gaseous or flammable material
Green	sewers and drain lines
Blue	drinking water
Purple	reclaimed water, irrigation, and slurry lines
Pink	temporary survey markings, unknown/unidentified facilities
White	proposed excavation limits or route

Some municipalities use the pink paint to make lines and codes on the pavements related to required street improvements such as ramp replacement, asphalt grinding and form injection. These markings are not related to utility locating.[16]

• DigNet, coordinators of digging activities
• Dynatel, a product line of utility locators and markers
• Subsurface utility engineering
• Underground Service Alert

• Technology Transfer Information Center. "Statement of Need: Utility Locating Technologies". Accessed October 31, 2007.

• American Public Works Association, an organization of public works professionals
• National Utility Locating Contractors Association, a utility locating organization